PURPOSE: To prevent the malfunction of a processor by reading a control data repetitively from a digital signal processor by a central processing unit to discriminate the presence of a data error, activating a mute circuit in the presence of the abnormality and transferring the same control data to the signal processor again.

CONSTITUTION: A CPU 6 reads repetitively the internal data of a digital signal processor DSP 2 and collates the data transferring the control data. When they are discondant with each other, it is discriminated that the internal data in the DSP 2 is abnormal, a mute circuit 4 is activated to prevent the output of a D/A converter 3 from being delivered to a speaker. Simultaneously, the same control data is transferred again to the DSP 2 to replace the internal data of the DSP 2 with the correct data. When the internal data is replaced into the correct data, since they are coincident with each other when the CPU 6 collates the internal data of the DSP 2 succeedingly, the mute circuit 4 is released and the output of the D/A converter 3 reaches the speaker.